Several electron-capture decaying radionuclides are widely used for imaging in Nuclear Medicine. These radionuclides emit low and very low energy electrons which may cause extensive damage in the biological systems as reported in the literature in case of 125I. It is proposed to study the radiotoxicity of several commonly used nuclear medicine radionuclides in vivo in experimental mice. Spermatogenesis will be used as a model to observe any excessive damage caused by the low energy electrons emitted by the incorporated radionuclides. Comparison with 137Cs external radiation and/or Beta-decaying radionuclides of the same element will give relative biological effectiveness of the incorporated radionuclides which may be useful to better estimate the radiation dose to the patient from the clinical procedures. The experimentally observed RBE values, which are expected to be considerably higher than 1 if highly localized energy deposition by Auger electrons is playing an important role, may be understood by autoradiographic studies and by calculating the microscopic dose to the radiosensitive volume.